base: unix: timer: use duration_to_timespec()
base: unix: timer: use duration_to_timespec()

Replace the duplicated Duration to timespec conversion code with calls
to the existing helper function.

BUG=None
TEST=cargo test -p base timer -- --include-ignored

Change-Id: I99d9e77c2056c760c37fd014ca99363276a473d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3652892
Reviewed-by: Anton Romanov <7c8c28e1283c1d87f2c3df4d6b5c74b6ae02bcd9@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm